Numerous aerobic processes have been developed over the years for the biological treatment of municipal waste which includes both domestic and industrial sewage for yielding an environmentally acceptable effluent. One of the widely-used aerobic processes for such treatment is referred to as the activated sludge process wherein organic matter contained in the municipal waste is contacted with an oxygen-containing gas in the presence of suspended biologically active organisms under conditions such that the organic material is converted into a form which can be separated from purified water. A portion of the insoluble sludge that is formed is recycled to the aerobic zone. Another is the trickling filtration method wherein the microorganisms are fixed to a support.
As is known in activated sludge systems and other aerobic processes, e.g., the trickling filtration process, there is usually a significant net positive production of sludge containing suspended solids in the process and there is an increasing inventory of sludge. Excess sludge must be discarded on a periodic basis from the process. Biological sludges produced by the activated sludge process and other aerobic processes are difficult and expensive to treat because these sludges have poor dewatering properties and are highly putrescible. Because of these characteristics, sludge disposition has become an important problem in the environmental conservation area. Ocean dumping of sludge or use as landfill are objectionable in the environmental conservation arena because such sludges present health hazards to the environment. Prior to disposal, these sludges require pasteurization so that the concentration of pathogenic organisms is sufficiently low to avoid potential health hazards.
Numerous processes have been developed for sludge stabilization and one process traditionally used has been anaerobic digestion. In anaerobic processes, the organic material present in the sludge is oxidized to by-products such as organic acids, ammonia, and principally methane. One of the problems associated with anaerobic digestion is that it is high cost in operation and substantial time is required for the digestion process. Thus equipment tends to be of a large scale.
Another process for stabilizing activated sludge is referred to as extended aeration wherein the sludge is contacted in an aerobic digestion zone and the organic material oxidized over time. Although extended aeration may offer significant advantages over anaerobic digestion, there are problems associated with such processes because of high operating expenses and capital costs associated with the extended residence time required to effect biological oxidation.
There are many variations in processes associated with the aerobic treatment of municipal waste which includes an activated sludge concept or alternate aerobic processes for handling the net for excess production of sludge. Such processes are described in the following references and these include:
U.S. Pat. Nos. 3,547,814 and 3,670,887 disclose the treatment of sewage wherein gross solids are first removed from the sewage by screening and the remaining waste contacted with an oxygen-containing gas and activated sludge. The '814 patent discloses that anaerobic processes have been used to render the sludge nonputrescible and as noted require long-term storage, e.g., 30 days, etc. Even after such treatment, the residual solids from the process comprise from 40-50% of the original volume of excess sludge. Another suggested technique for treating such sludge involved extended aeration which increased the degree of auto-oxidation, i.e., the sludge became self-consuming to some degree, and there was a net reduction of such sludge. Unfortunately, the rate of oxidation was generally too low to have a significant effect on net sludge production. Even with extended aeration and increased degree of auto-oxidation, particularly at the zero net production of sludge level, problems were presented because of large plant size and high operating costs. For example, capacities were three to six times larger than a conventional activated sludge plant. To reduce size, the patentees suggested using an oxygen-rich gas and a high volatile suspended solids concentration in the sludge to oxidize organic material in the sludge. This resulted in a low sludge yield in the overall process.
U.S. Pat. No. 3,356,609 discloses a process for treating municipal waste wherein the initial sewage is clarified generating a bottoms fraction or raw sludge and effluent containing suspended or soluble organic matter. The effluent is then enriched with a carbon source and contacted with an oxygen-containing gas and activated sludge in a dispersed culture aerobic reactor. The product from the dispersed culture aerobic reactor is contacted with a flocculant and separated. The sludge formed then is separated in a secondary clarifier and a portion recycled to the aerobic reactor and the excess, along with the raw sludge from the primary clarifier, oxidized in a sludge aerobic reactor.
U.S. Pat. No. 4,246,099 discloses a combination of aerobic/anaerobic concepts to reduce and stabilize sludge solids in an activated sludge process. In this process, municipal sludge was initially contacted with an oxygen-containing gas under aerobic conditions to partially reduce the biodegradable volatile suspended solids and then anaerobically digested to partially-stabilize the sludge. Sludge reduction to less than 40% of the biodegradable volatile suspended solids introduced to the digestion zone was achieved. The concept disclosed for aerobic digestion was referred to as autothermal aerobic digestion wherein the digester was operated at elevated temperatures, e.g., from about 45.degree.-75.degree. C. or the thermophilic range. As the patentees indicated, the mesophilic microorganism population declined while the thermophilic forms increased and the rate of biological digestion increased at these higher temperatures.
U.S. Pat. No. 4,026,793 discloses an aerobic digestion process for reducing the solids content in a biodegradable organic sludge by carrying out the digestion in a vessel maintained at a temperature within the range of 38.degree.-46.degree. C. This temperature was alleged to enhance the development of protozoa Monodidae and thus enhance the reduction of solids in the sludge.
U.S. Pat. 4,652,374 discloses a modified anaerobic fermentation of municipal waste by effecting hydrolysis and acidification of the sewage and then anaerobically digesting the hydrolyzed sewage under conditions for methane generation.
It is also known in a modified extended aeration, activated sludge process in combination with autothermal aerobic digestion (ATAD) to use a hydrolytic assist which comprised the treatment of the effluent from the ATAD with acid and subjecting the resulting hydrolyzed effluent to biological digestion in the initial aeration zone wherein the sewage was contacted with an oxygen-containing gas and activated sludge. Proceedings, 40th Annual Purdue Industrial Waste Conference, Ann Arbor Press, Ann Arbor, MI, pp. 775-784 (1986).
As can be seen from the review of substantial prior art pertaining to aerobic processes, including activated sludge processes, many variations have been proposed in an effort to reduce or minimize sludge production and to stabilize excess sludge produced by aerobic processes. All of these processes in one way or another become quite complex and may exhibit high operating costs or capital costs in order to achieve that objective. In most cases, it is extremely difficult to modify these processes in such a way that there is substantial sludge reduction, based on original organic input, let alone achieving sludge elimination. The latter goal is one often sought but seldom achieved and typically requires intervening physical separation processes such as dewatering and subsequent incineration. Removal of organics from waste streams via respiration and conversion into microbial mass and its subsequent conversion to water and carbon dioxide is seldom achieved.